/* Ascending priority queue using Linked List - Program to implement Ascending priority queue using Linked List */

/*A priority queue is a special type of queue in which each element is associated with a priority and is served according 
to its priority. If elements with the same priority occur, they are served according to their order in the queue.

Generally, the value of the element itself is considered for assigning the priority.

For example: The element with the highest value is considered as the highest priority element. However, in other cases, 
we can assume the element with the lowest value as the highest priority element. In other cases, 
we can set priorities according to our needs.

In a queue, the first-in-first-out rule is implemented whereas, in a priority queue, the values are removed on the basis of priority. 
The element with the highest priority is removed first.

insert() - Would insert an element in a queue 
delete() -  Would delete the smallest element in the queue
*/


#include  <stdio.h>
#include  <stdlib.h>  
#define  NULL  ((void*)0)

struct  node
{
  int  data ;
  struct  node  *next ;
} ;

struct  node  *front , *rear ;

/* This function initializes the queue to empty by making both front and rear as NULL */
void createqueue()
{
  front=rear=NULL ;
}

int  empty()
{
  if(front==NULL)
    return  1 ;
  else
    return  0 ;
}

void  insert(int  x)
{
  struct  node  *pnode ;

  pnode=(struct node*)malloc(sizeof(struct node)) ;
  if(pnode==NULL)
  {
    printf("Memory overflow. Unable to insert.\n") ;
    exit(1) ;
  }

  pnode->data=x ;  
  pnode->next=NULL;   /* New node is always last node */

  if(empty())
      front=rear=pnode ;
  else
  {
      rear->next=pnode ;
      rear=pnode ;
  }
}

int  removes()
{
  int  min ;
  struct  node  *follow , *follow1 , *p , *p1 ;

  if(empty())
  {
    printf("\nQueue Underflow. Unable to remove.") ;
    exit(1) ;
  }

  /* finding the node with minimum value in the APQ.*/
  p=p1=front ;
  follow=follow1=NULL ;
  min=front->data ;
  while(p!=NULL)
  {
    if(p->data<min)
    {
   min=p->data ;
   follow1=follow ;
   p1=p ;
    }
    follow=p ;
    p=p->next ;
  }

  /* Deleting the node with min value */

  if(p1==front)  /* deleting first node.*/
  {
    front=front->next ;
    if(front==NULL)  /* Deleting the only one node */
      rear=NULL ;
  }
  else if(p1==rear)  /* Deleting last node */
  {
      rear=follow1 ;
      rear->next=NULL ;
  }
  else      /* deleting any other node.*/
      follow1->next=p1->next ;


  free(p1) ;
  return  min ;  /* DONT FORGET LAST 2 STATEMENTS.*/
}

void  show()
{
  struct  node  *p ;

  if(empty())
    printf("Queue empty. No data to display \n") ;
  else
  {
    printf("Queue from front to rear is as shown: \n") ;

    p=front ;
    while(p!=NULL)
    {
      printf("%d ",p->data) ;
      p=p->next ;
    }

    printf("\n") ;
  }
}

void  destroyqueue()
{
  front=rear=NULL ;
}

int  main()
{
  int  x , ch ;

  createqueue() ;


  do
  {
    printf("\n\n  Menu: \n") ;
    printf("1:Insert \n") ;
    printf("2:Remove \n") ;
    printf("3:exit \n") ;
    printf("Enter your choice: ") ;
    scanf("%d",&ch) ;

    switch(ch)
    {
      case 1:
      printf("Enter element to be inserted: ") ;
      scanf("%d",&x) ;
      insert(x) ;
      show() ;
      break ;

      case 2:
      x=removes() ;
      printf("Element removed is: %d\n",x) ;
      show() ;
      break ;

      case 3:
      break ;
    }
  }
  while(ch!=3) ;

  destroyqueue() ;

  return 0;
}

/* Output of the Program*/

/*
  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 1
Enter element to be inserted: 12
Queue from front to rear is as shown:
12


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 1
Enter element to be inserted: 1
Queue from front to rear is as shown:
12 1


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 1
Enter element to be inserted: 14
Queue from front to rear is as shown:
12 1 14


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 1
Enter element to be inserted: 3
Queue from front to rear is as shown:
12 1 14 3


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 1
Enter element to be inserted: 5
Queue from front to rear is as shown:
12 1 14 3 5


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 2
Element removed is: 1
Queue from front to rear is as shown:
12 14 3 5


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 2
Element removed is: 3
Queue from front to rear is as shown:
12 14 5


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 2
Element removed is: 5
Queue from front to rear is as shown:
12 14


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 2
Element removed is: 12
Queue from front to rear is as shown:
14


  Menu:
1:Insert
2:Remove
3:exit
Enter your choice: 2
Element removed is: 14
Queue empty. No data to display

*/
